20 - 30 Minuten
I got an old iPhone 3Gs from work to mess around with. Then after a few days the screen popped out as the battery had gone off. It was as bloated as a corpse in a river.
Repair went easily and well. The guide was great as per usual. My only issue was I put the new battery in upside down and too 10 minutes to figure out why I couldn't put the phone back together.
When I got the 3Gs back running I put iOS6 on and it is running fine. Now I have a spare phone and one I can use for internationa visitors.